This invention relates generally to optical beam steering systems and, more particularly, to a two-dimensional, phased-array beam steerer for use in a laser radar system.
There are currently very pressing needs for rapid, large-angle pointing and scanning of laser beams of all wavelengths from the infrared through the ultraviolet. These needs include strictly military applications such as laser weapons, which require agile, high-energy laser pointing, and laser radar, used for target search, acquisition, tracking and surveillance. There are also purely commercial needs such as laser light shows and laser printing, which require rapid, programmable beam steering. In addition, there are also many areas common to both military and commercial interests such as optical computing and image processing, which require rapid scanning of spatial light modulators, and optical data storage requiring rapid optical addressing. In most of these cases, the impediment to effective performance of the optical system is in the area of beam steering.
Presently available technologies are generally not sufficiently advanced to supply the need for rapid, large-angle pointing and scanning of optical beams and, in particular, of large diameter, diffraction-limited carbon dioxide (CO.sub.2) laser radar beams. In many systems, optical beam steering is currently performed using rotating optical elements. Such systems typically consist of galvanometer mirrors and a focal telescopes, performance being limited to beam diameters of somewhat less than six inches, a field of view of approximately five degrees in each direction, and a frame time of approximately one second with a few thousand resolution cells and open-loop, random-access time on the order of ten milliseconds. The capability of handling larger beams is required for higher power systems, particularly for many of the military applications for laser radar systems. Larger fields of view and larger apertures, on the order of one-half to one meter diameter, are of great interest, and faster scan times are desired for many applications. In short, there exists a pressing need for an optical version of the versatile phased-array antennas now widely used for microwave radar systems.
A static deflector for deflecting a polarized infrared beam is suggested by U.S. Pat. No. 4,639,091, issued Jan. 27, 1987, to J.-P. Huignard et al. The Huignard et al. deflector comprises a layered square plate having as a front layer a window on which stripe electrodes are disposed. Both the window and the stripe electrodes are transparent to an incident infrared beam. A middle layer of the deflector comprises an electro-optical liquid crystal layer. The bottom layer comprises a substrate having a common electrode adjacent to the liquid crystal layer. The common electrode is preferably reflective at the beam wavelength; illustratively, it is a gold film. Alternatively, for a deflector operating by transmission, a transparent rear plate may be used.
Huignard et al. and others in the microwave phased-array antenna arts have suggested a periodic staircase waveform comprising N voltage steps which are applied to the stripe electrodes, thereby creating local variations of the refractive index in the liquid crystal layer in such a manner as to form a beam diffraction grating of adjustable period.
The specific application which supplies the impetus for the present invention is a laser radar system. Such a system requires optical beam steerers which provide rapid, non-mechanical, random-access pointing of large optical beams, on the order of one meter diameter. The requirement of random-access pointing necessitates deflection of the beam along two dimensions. Huignard et al. address the concept of two-directional X-Y deflection of a beam by suggesting that two static deflectors be assembled having their control electrodes at 90.degree. to each other. Alternatively, Huignard et al. suggest using a matrix of individually addressable points rather than strip electrodes.
However, due to the precious nature of aperture area in radar-bearing vehicles (aircraft, missiles, satellites, tanks, etc.), existing laser radar transceivers typically multiplex a single output aperture between transmitter and receiver by using orthogonal circular polarizations on the separate channels. A difficulty arises, then, in employing one or more liquid crystal arrays to point the beam, since the operation of currently known, liquid crystal phased arrays requires linear polarization.